Interferometric Performance and Controls Automation for the TorPeDO Dual Torsion Pendulum Gravitational Sensor

Abstract

The era of gravitational-wave astronomy began with the first direct detection of gravitational-waves, in 2015. Highlights from the first three observing runs include the key role that binary neutron star mergers play in nucleosynthesis, and probing the lightest intermediate-mass black holes. These successes have been achieved with three, second generation, advanced gravitational wave detectors both Advanced LIGOs, and Advanced Virgo.

Capitalising on these successes, design work has begun for the third generation of gravitational-wave observatories. These will achieve greater sensitivities and bandwidths, permitting a larger volume of the universe to be probed and greater signal-to-noise ratio detection to be made. An unsolved challenge, limiting the performance of these planned observatories, is direct gravitational coupling from local environmental fluctuations - Newtonian noise.

Applying the same technologies, which enabled the direct detection of gravitational-waves, is one method being investigated to subtract Newtonian noise from future observatories' readouts. This is one major goal of suspended, interferometric, dual torsion pendula sensors. Torsion pendula act as free masses, at significantly lower frequencies than the suspensions of a gravitational-wave observatory. Interferometrically measuring the displacement between two torsion pendula, gravitationally induced by Newtonian noise, provides a means to subtract all sources of this noise from an observatory's gravitational-wave readout.

This thesis focuses on the Torsion Pendulum Dual Oscillator (TorPeDO) sensor; one example of these suspended, interferometric, dual torsion pendula sensors, under development at The Australian National University's, Centre for Gravitational Astrophysics. Specifically this thesis focuses on a control scheme for the TorPeDO's interferometric readout, different to previously investigated schemes, and compatible with future upgrades to the sensor's seismic isolation. Alongside this results from two pieces of commissioning work, undertaken for the Advanced LIGO Livingston Observatory during observing run three, are presented.

The first Advanced LIGO commissioning contribution is modelling to improve damping for some of the primary optics suspensions. This additional feedback removes energy faster than global alignment controls injects it, stabilising this actuation. Additional damping permitted global alignment controls to return to their natural basis, and was deployed at both Advanced LIGO detectors. The second commissioning contribution is automation of the initial alignment procedure. This includes initial investigations, presented here, into halting these routines based upon measurements of the alignment.

Leveraging the same tools as Advanced LIGO, the TorPeDO's new interferometric control scheme was robustly demonstrated by automating lock acquisition. This permitted a measurement understanding of the TorPeDO controls prototype's performance. Differential rotation of 40 nrad Hz^-1/2 at 0.1 Hz was measured, in-air and without seismic isolation.

This thesis concludes with proposals to improve the performance of the TorPeDO, considerations for the commissioning of its seismic isolation, and recommendations for suspensions in gravitational-wave observatories. These include further lines of investigation which should be pursued to increase the TorPeDO's sensitivity. Show more